PROJECT SUMMARY Glioblastomas (GBMs), the most common and deadly primary malignant brain tumors, frequently display mutations that activate receptor tyrosine kinases (RTKs) and the Pi-3 kinase-Akt (PI3K) signaling pathway. Yet, to date, drugs that directly inhibit RTK and/or PI3K signaling have proven ineffective in treatment of GBM. Our research program uses complementary experimental model systems, including patient-derived primary GBM stem cell cultures and novel GBM models in Drosophila melanogaster, to identify and interrogate genetic and cellular processes that underlie glial tumorigenesis. Using our GBM model systems, we identified RIOK2, a highly conserved atypical serine-threonine kinase, and our results revealed that, in GBMs, RIOK2 becomes overexpressed in response to RTK-PI3K signaling to drive tumor cell proliferation and survival, and that RIOK2 knockdown in RTK-PI3K dependent GBM cells inhibits proliferation and elicits apoptosis. Our preliminary data also indicate that, in both Drosophila and human GBM models, inhibition of RIOK2 catalytic activity strongly counteracts RTK-PI3K-driven transformation. In contrast, loss or inhibition of RIOK2 activity in normal human or Drosophila glial progenitor/stem cells does not elicit cell death and does not dramatically affect cell proliferation. Thus, RTK-PI3K mutant tumor cells, but not normal glial cells, are dependent on RIOK2 activity for their growth and survival. Based on our preliminary data, we hypothesize that RIOK2 drives GBM by phosphorylating and activating RNA-binding proteins to promote the translation of oncogenic target mRNAs, and that RIOK2 differentially promotes the growth and survival of GBM cells by stimulating the translation of oncogenic mRNAs that are expressed as a consequence of gliomagenic RTK and PI3K signaling. This proposal focuses two aims designed to Aim 1) Determine the mechanisms of interaction between RIOK2 and its associated RNA-binding proteins, and Aim 2) Determine if RIOK2 activity specifically promotes translation of select target mRNAs in tumor cells. This project may reveal new mechanisms that drive tumorigenesis, and establish a basis for developing RIOK2 inhibitors for potential targeted treatment for GBM.